1. Field of the Invention
This invention relates to apparatus for detecting unsymmetrical bipolar waveforms such as transformer in-rush currents and has particular application to preventing false trips in electric power transmission system protective relays.
2. Background Information
When a transformer in an electric power transmission system is energized with little or no load, unsymmetrical in-rush current which lasts for several minutes is produced. Such transient currents can have adverse effects on certain protective relays. For instance, they can cause false tripping of the reverse current relay described in U.S. Pat. No. 4,698,718. This relay monitors the phase relation between the current and voltage in a transmission line and opens a circuit breaker if this phase relationship indicates a reverse power condition. The reverse power condition is indicated by the presence of a negative dc component in the product of a signal representative of the current and a phase shifted signal representative of voltage. The pulses generated by transformer in-rush current produce a dc component in this product signal which is sufficient to trip the breaker.
It is an object of the present invention to provide apparatus for detecting unsymmetrical bipolar waveforms.
It is another object of the invention to provide such apparatus which acts rapidly to detect unsymmetry within one cycle of the waveform.
It is yet another object of the invention to provide such apparatus which can be easily adjusted to detect varying degrees of unsymmetry.
It is still another object of the invention to provide such apparatus which maintains an indication of unsymmetry until symmetry of the waveform is firmly reestablished.
It is also an object of the invention to provide apparatus as described in the previous objects which is adaptable for use in detecting unsymmetry in the waveforms of electric transmission lines before the unsymmetry causes false operation of a protective relay.